The present invention relates generally to an apparatus and method of mass spectrometry. More specifically, although not exclusively, this invention relates to a mass spectrometer and a method of mass spectrometry. Mass spectrometers configured for Matrix-Assisted Laser Desorption Ionisation (“MALDI”) are known. MALDI is a soft ionisation technique for mass spectrometry in which the analyte molecules are prepared on the surface of a target plate. They are supported in a solid polycrystalline matrix. A pulse of laser radiation, with a typical duration of a few nanoseconds, is directed onto the MALDI sample which is strongly absorbed by the matrix molecules. This pulse of laser energy results in rapid heating of the region that is irradiated. This heat causes a proportion of the matrix material to be vaporised and explosively ejected from the surface as a plume of gaseous material (desorption). Analyte ions, embedded within the matrix that is desorbed, are transferred to the gaseous phase along with the matrix. Reactions between the matrix ions and the analyte molecules can result in the analyte molecules being ionised either through protonation/deprotonation or through the removal or addition of an ion. Upon dispersal of the initial MALDI plume, the remaining analyte ions are predominantly singly charged.
Although the absorption of the laser radiation occurs at all levels of laser fluence, there is a threshold energy density required in order to obtain desorption of material under illumination.
MALDI imaging is a growing technique where the sample to be analysed may be a thin (typically 15 μm) section of tissue, with a layer of matrix deposited upon the surface. The sample is scanned in a raster manner, with the laser firing at specific locations or ranges of locations spaced along the raster pattern. Mass spectra are acquired at each location or range of locations and the relative abundance of ion masses is then displayed as an ion image of the tissue section. The image resolution to which the spatial distribution of ions can be determined is a function of the distance between each spectral location and the area of the sample irradiated above the ionisation threshold by each individual laser pulse. Therefore, the spatial resolution can be improved by the use of a small diameter laser intensity profile. A shorter distance from the final laser lens to the sample is therefore advantageous in improving the spatial resolution of the ion image.
In order to obtain a high spatial resolution of the MALDI source, the area irradiated by the laser pulse must be reduced in area. This is determined by several factors associated with the laser beam profile incident upon the focusing element, including the beam diameter and the beam profile. It is also determined by the focal length of the focusing optic, and hence the working distance between the lens and the MALDI sample plate. One further issue that determines the size of the laser pulse incident on the sample is the angle of incidence of the laser beam. With this in mind it is preferable to ensure that the laser beam is orthogonally incident upon the sample target.
The plume and analyte ions formed by irradiation by the laser tends to expand in a direction towards the incident laser beam. This is because of the inhomogeneous surface topography of the MALDI sample and crystalline matrix. Reference is made to P. Aksouh et al. Rapid Commun. Mass Spectrometry, 9 (1995) 515.
The ions formed in the MALDI plume must be transferred into the analyser. This requires electrodes to be located in close proximity to the sample target. In high vacuum MALDI instruments, the requirement for electrostatic lenses to be also arranged along the ion optic axis to enable ion acceleration orthogonal to the sample plate generally precludes the ability to locate laser optics along the same path. Consequently, many MALDI mass spectrometers are designed with the laser incident at a small but non-zero angle of incidence. For other systems with orthogonal illumination electrostatic deflectors have been used to guide ions around the laser optics.
With intermediate pressure MALDI, where a hexapole RF guide is used to transfer ions, the RF device prevents the possibility of locating laser optics designed specifically to provide orthogonal illumination. Furthermore, the RF lenses limit the possibility of providing a final focus lens close to the MALDI sample plate. Similar constraints also apply to atmospheric pressure MALDI instrumentation.
It is desired to provide an improved mass spectrometer and method of mass spectrometry.